Artifact suppression in multi-coil MRI

ABSTRACT

A magnetic resonance imaging system ( 1 ), comprising a plurality of receiving units ( 4.1 - 4.4 ) for receiving magnetic resonance signals from an object ( 2 ), and an image reconstruction device ( 8 ), said image reconstruction device being adapted to receive magnetic resonance signals of said object ( 2 ) from said plurality of receiving units ( 4.1 - 4.4 ) and to perform image reconstruction by combining magnetic resonance signals received by said plurality of receiving units using an image reconstruction algorithm ( 11 ), characterized in that said image reconstruction device ( 8 ) comprises means ( 12   a ) for combining magnetic resonance signal contributions from respective receiving units ( 4.1 - 4.4 ) in such a way that a combined sensitivity of the plurality of receiving units ( 4.1 - 4.4 ) to a predetermined spatial region of the object ( 2 ) is reduced.

CROSS REFERENCE TO PRIOR APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/374,321 filed on Jan. 19, 2009, which is the U.S. National Stageapplication of International Application No. PCT/IB2007/052588 filed onJul. 3, 2007,now U.S. Pat. No. 8,244,011, which claims priority to EP06117393.6 filed on Jul. 18, 2006. These applications are incorporatedby reference herein in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic resonance imaging system,comprising a plurality of receiving units for receiving magneticresonance signals from an object, further comprising an imagereconstruction device, said image reconstruction device being adapted toreceive magnetic resonance signals of said object from said plurality ofreceiving units and to perform image reconstruction by combiningmagnetic resonance signals received by said plurality of receiving unitsusing an image reconstruction algorithm.

The present invention also relates to a method of image reconstructionin a magnetic resonance imaging system, comprising receiving magneticresonance signals of an object from a plurality of receiving units andperforming image reconstruction by combining magnetic resonance signalsreceived by said plurality of receiving units by means of an imagereconstruction algorithm.

The present invention further relates to a computer program product,particularly for storing on a computer-readable storage means, for usein a magnetic resonance imaging system, and more particularly forupgrading an existing magnetic resonance imaging system, said computerprogram product comprising first code sequences for implementing amechanism for receiving magnetic resonance signals of an object from aplurality of receiving units and second code sequences for implementingan image reconstruction algorithm performing image reconstruction bycombining magnetic resonance signals received by said plurality ofreceiving units.

In magnetic resonance imaging (MRI) it frequently happens that a regionwhich is not of clinical interest disseminates artefacts over clinicallyinteresting regions. For instance, the aorta may disseminate flowartefacts, i.e., a particular form of motion artefacts, onto the liver.In much the same way, the breast or the heart may cause artefacts overthe spine due to their respective motions.

A prior art approach to obviating generation of the above-mentionedartefacts consists in employing a Regional Saturation Technique (REST)which involves applying a so-called saturation pulse to a problematic,i.e. moving area of an object to be imaged, in particular a human body.This technique is well known in the art and effectively prevents objectatoms in said problematic area to emit magnetic resonance signals.

However, said technique is often bound to geometrical restrictions,because performing pre-saturation in regions other than straight slabsis usually impractical. Furthermore, the above-described prior artapproach suffers from serious drawbacks in terms of scan time,attainable repetition rate, etc. In addition, due to said saturationpulses an amount of radiation absorbed by the patient is increased.

OBJECT OF THE INVENTION

It is the object of the present invention to provide a magneticresonance imaging system as well as a method of the above-mentioned typewhich enable obtaining high-quality magnetic resonance images from anobject, e.g. a patient, which do not suffer from artefacts caused by anykind of instability, e.g. motion or other local effects, which occurwith respect to a limited portion of an object under study, and whichare not bound to geometrical restrictions.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention the object isachieved by providing a magnetic resonance imaging system of theabove-mentioned type, wherein said image reconstruction device comprisesmeans for combining magnetic resonance signal contributions fromrespective receiving units in such a way that a combined sensitivity ofthe plurality of receiving units to a predetermined spatial region ofthe object is reduced.

According to a second aspect of the present invention the object isachieved by providing a computer program product of the above-mentionedtype which comprises further code sequences for implementing a mechanismfor combining magnetic resonance signal contributions from respectivereceiving units in such a way that a combined sensitivity of theplurality of receiving units to a predetermined spatial region of theobject is reduced.

According to a third aspect of the present invention the object isachieved by providing a method of the above-mentioned type whichcomprises the step of combining magnetic resonance signal contributionsfrom respective receiving units in such a way that a combinedsensitivity of the plurality of receiving units to a predeterminedspatial region of the object is reduced.

Thus, by employing a magnetic resonance signal acquisition systemcomprising a plurality of receiving elements, which may also be used toperform parallel Magnetic Resonance Imaging (pMRI), the problem ofartefact images can be solved by simply modifying the imagereconstruction or coil combination algorithm such that the result ofsaid algorithm shows minimum sensitivity to a predefined region or areaof the object which is known to be problematic with respect to imagereconstruction, for instance due to intrinsic motion of said region orarea. In this way, magnetic resonance imaging, e.g. parallel magneticresonance imaging, allows for intrinsically avoiding image artefacts.

For instance, assume a first coil located in front of the object(patient) and a second coil located behind the object (patient). Saidfirst coil has a spatial sensitivity of 2 (in arbitrary units) at afirst region of the object (e.g. the patient's heart) and a spatialsensitivity of 1 at a second region of the object (e.g. the patient'sspine). The second coil has a spatial sensitivity of 2 (in arbitraryunits) at the first region of the object and a spatial sensitivity of 2at the second region of the object. If the second region (the spine) isof interest for a particular study, then in accordance with anembodiment of the present invention the first and second coils, i.e.their respective signal contributions, should be combined subtractively,thus yielding zero sensitivity, i.e. a reduced or relatively lowersensitivity, at the first region of the object, which is presumed todisseminates artefacts over the more interesting second region.

In one embodiment of the invention a user may indicate an area known forcausing motion or other kinds of artefacts. This approach has somesimilarity to placing REST slabs known from the prior art, whereinplacing of said slabs is also planned by the user. However, sinceartefacts are avoided by modification of the image reconstructionalgorithm only, there is no restriction of region geometry to slab-likestructures. Furthermore, assuming that acquired magnetic resonance rawdata is kept in memory, defining said region can be done either beforeor after the actual acquisition process, thus making the inventiveapproach highly flexible during practical operation. Additionally, theavoidance of REST slabs has the advantage of reducing the amount ofradiated power to which an object, e.g. a patient, is exposed.

In a corresponding embodiment of the computer program product inaccordance with the present invention the latter further comprises codesequences for implementing a mechanism for receiving input data forgeometrically defining said region before or after measuring saidmagnetic resonance signals.

In one embodiment of the magnetic resonance imaging system in accordancewith the present invention, said image reconstruction device of thelatter comprises means for estimating image data points p by evaluatingthe expressionp=(S ^(h)Ψ′⁻¹ S)⁻¹ S ^(h)Ψ′⁻¹ m,  (Eq. 1)with Ψ′=Ψ+Ψ_(instability), wherein m is a vector with measured magneticresonance data per receiving unit, S is an array of receiving unit orcoil sensitivities, S^(h) is the hermitian transpose of S, Ψ a noisecovariance matrix, and Ψ_(instability) is an additional term accountingfor a motion or any kind of local instability of the object.Departing from the known image reconstruction algorithmp=(S ^(h)Ψ⁻¹ S)⁻¹ S ^(h)Ψ⁻¹ m,this provides for improving the prior art as desired. Furthermore, thisapproach particularly lends itself for upgrading an existing magneticresonance imaging system, e.g. by providing and implementing suitablesoftware modules.

Said approach is valid for parallel imaging as well. Then, in analogy toPruessmann et al. (Magn. Reson. Med. 1999; 42: pp. 952-962), p becomes avector and S becomes a matrix.

In a corresponding embodiment of the computer program product inaccordance with the present invention the latter further comprisesprogram code sequences for implementing a mechanism for estimating imagedata points by evaluating the above-defined Eq. 1.

In a corresponding further embodiment, the method in accordance with thepresent invention further comprises estimating image data points byevaluating the above-defined Eq. 1.

In yet another embodiment of the magnetic resonance imaging system inaccordance with the present invention, said image reconstruction deviceof the latter comprises means for determining said additional term(matrix Ψ_(instability)) according to the relation:Ψ_(instability,ik) =a·1/A·Σ _(area) s _(i)(x,y)s_(k)(x,y)∥q(x,y)∥²,  (Eq. 2)wherein Ψ_(instability,ik) are matrix elements of said additional term,A is a quantity of said predetermined spatial region of the object,q(x,y) is a signal strength of a reference signal, s_(i)(x,y),s_(k)(x,y) is a sensitivity of receiving unit i, k at image position(x,y), respectively, and a is a predefined numerical factor.Furthermore, x and y denote spatial coordinates in the 2D image plane.

In a further embodiment of the computer program product in accordancewith the present invention the latter further comprises code sequencesfor implementing a mechanism for determining said additional termaccording to the above-defined Eq. 2. In another embodiment the computerprogram product in accordance with the present invention may furthercomprise program code sequences for implementing a mechanism forincluding said additional term in said image reconstruction algorithm.

In a corresponding embodiment, the method in accordance with the presentinvention further comprises determining said additional term accordingto the above-defined Eq. 2. In another embodiment the method inaccordance with the present invention may further comprise includingsaid additional term in said image reconstruction algorithm.

Using the inventive approach in connection with pMRI further reducesscan time and the amount of radiation absorbed by the object (patient).

Further advantages and characteristics of the present invention can begathered from the following description of preferred embodiments withreference to the appended drawings. Features mentioned above as well asbelow can be used either individually or in conjunction in the contextof the present invention. The described embodiments are not to beregarded as an exhaustive enumeration but rather as examples withrespect to a basic concept underlying the present invention.

Note that while embodiments of the present invention will hereinafter bedescribed primarily with respect to avoiding motion artefacts, any otherkind of artefact caused by local instabilities could also be avoided ina similar fashion. Beside the above-mentioned flow artefacts, systeminstabilities caused by a pronounced off-centre location of some part ofan object under study or by differences in brightness of some part ofthe object may be of relevance here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a magnetic resonance imagingsystem comprising an image construction device in accordance with thepresent invention;

FIG. 2 is a schematic diagram for illustrating the effect of heartmotion troubling spine-imaging;

FIG. 3 is a schematic diagram illustrating a defined region forobviating motion-induced artefacts in accordance with the presentinvention; and

FIG. 4 is a flow chart for illustrating an embodiment of the method inaccordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a magnetic resonance imaging system 1 inaccordance with the present invention. For providing magnetic resonanceimages of an object 2, e.g. a patient, said system 1 comprises a mainmagnet 3 producing a strong magnetic field in which object 2 is placedfor magnetic resonance imaging, as known to a person skilled in the art.Said magnetic field is a uniform magnetic field that causes an alignmentof the moments of magnetic spin of atoms contained within the object. Asfurther known to a person skilled in the art, system 1 further comprisesa number of functional units (not shown) that apply a transversemagnetic field, generated by radio frequency (RF) pulses, to the objectsuch that the aligned moments rotate or tip, thereby exciting the spinsof the atoms. The excited spins of the atoms generate a magneticresonance signal that is detected by a plurality of receiving units4.1-4.4 (receiving or imaging coils) comprised within the magneticresonance imaging system 1.

The magnetic resonance imaging system 1 of FIG. 1 further comprises acontrol unit 5 which is operatively connected with an input unit 6, andwhich generally comprises storage means 7 and an image reconstructiondevice 8 (dashed box in FIG. 1). The latter is preferably devised insoftware form, e.g. by implementing suitable program code sequences onsaid control unit 5.

Image reconstruction device 8 comprises first receiving means 9 forreceiving said magnetic resonance signals of object 2 from saidreceiving units 4.1-4.4. Image reconstruction device 8 further comprisessecond receiving means 10 for receiving input data from input unit 6.Image reconstruction device 8 further includes an algorithmic unit 11having further means 12, 12 a, and 13, a function of which will bedescribed in detail below.

As known to a person skilled in the art, during operation of themagnetic resonance imaging system 1 of FIG. 1, receiving units 4.1-4.4provide magnetic resonance data of object 2. Said data, i.e.corresponding magnetic resonance signals, are received by firstreceiving means 9 of control unit 5/image reconstruction device 8. Usingmultiple receiver coils 4.1-4.4 each with different (and known) spatialsensitivities allows reconstructing of the magnetic resonance image ofobject 2 by employing a suitable image reconstruction algorithm asprovided by algorithmic unit 11 of FIG. 1. To this end, data received byfirst receiving means 9 preferably is first stored (at leasttemporarily) in storage means 7 and then provided to algorithmic unit 11for image reconstruction.

Image reconstruction is based on the above-defined algorithm (e.g., asdefined by Eqs. 1 and 2) which is implemented by means of algorithmicunit 11 for operating on said magnetic resonance signal raw dataprovided by first receiving means 9 and/or storage means 7, aspreviously described.

As already stated above, a suitable image reconstruction algorithm canbe written as:p=(S ^(h)Ψ⁻¹ S)⁻¹ S ^(h)Ψ⁻¹ m,wherein m is a vector with the measured data per coil element i(4.1-4.4), p is an estimated of image data point (or a vector of imagedata points in the case of parallel imaging), S is an array of coilsensitivities matrix (s_(i)(x,y) being the sensitivity of coil i atimage position (x, y)), S^(h) is the hermitian transpose (transposedcomplex conjugate) of S, and Ψ is the noise covariance matrix (Ψ_(ii)being the square of the noise deviation of coil element i).

As already pointed out the above-defined reconstruction algorithmgenerally can be used for parallel and non-parallel imaging, whereinnon-parallel imaging is regarded as a limiting case with a speed-upfactor equal to unity. In the case of non-parallel imaging, p is ascalar and S is a 1*N matrix, N being the number of receiving units(coils) used. As for parallel imaging, Ψ is an N*N matrix.

As already stated above, in MRI it frequently happens that a region thatis not of (clinical) interest disseminates artefact over (clinically)interesting regions of an object to be studied. In medical MRI, onetypical example is the aorta which disseminates flow artefacts onto theliver. Another typical example is the breast or the heart which—due totheir respective motion—may cause artefacts over the spine. The lattercase is schematically illustrated in FIG. 2:

FIG. 2 schematically shows a sagittal view through a human body 14including the spine 15, which presently is of clinical interest, and theheart 16, which is not of interest in that particular study.

When using the magnetic resonance imaging system 1 as described so farwith reference to appended FIG. 1 motion artefact data in the magneticresonance signals provided by receiving units 4.1-4.4 (FIG. 1) wouldresult in motion artefacts being present in the reconstructed image dataprovided by image reconstruction device 8, i.e. algorithmic unit 11.

In order to obviate this effect, the image reconstruction device8/algorithmic unit 11 of the magnetic resonance imaging system 1 inaccordance with the present invention further comprises (software) means12 a for combining magnetic resonance signal contributions fromrespective receiving units 4.1-4.4 in such a way that a combinedsensitivity of the plurality of receiving units is relatively lower,i.e. reduced, to a predetermined spatial region of the object 2, aspreviously described in connection with first and second receiving coilslocated in front of and behind an object, respectively. In other words:Said receiving coils are combined in such a way that the combinationshows little or no sensitivity to an area of the object, in particularto an area which is suspected to cause artefacts in the resultingmagnetic resonance image.

Alternatively or additionally, the image reconstruction device8/algorithmic unit 11 of the magnetic resonance imaging system 1 inaccordance with the present invention may comprise (software) means 12for including in the above-defined image reconstruction algorithm anadditional term Ψ_(instability) which accounts for a motion of apredetermined spatial region of object 2. Furthermore, according to theembodiment of FIG. 1, image reconstruction device 8/algorithmic unit 11further comprises means 13 for determining said additional matrix termΨ_(instability) according to the relation:Ψ_(instability,ik) =a·1/A·Σ _(area) s _(i)(x,y)s_(k)(x,y)∥q(x,y)∥²,  (Eq. 2)wherein Ψ_(instability,ik) is a matrix element of said extra termΨ_(instability), A refers to an area of said predetermined spatialregion (region of motion) of the object, and a is a (user-defined)factor indicating an expected quantitative amount of motion artefacts.The value of a should be chosen in accordance with a “motion quality” ofthe moving object, e.g. the heart 16 in the example of FIG. 2, andshould be essentially unity if the moving object is known to be veryjerky and unpredictable. Furthermore, q(x,y) is an amount of signal(signal strength) measured by a reference coil (not shown in FIG. 1)during a reference scan for correctly adjusting an algorithmical impactof the proposed additional term Ψ_(instability) in accordance with thepresent invention.

In this way, the reconstruction algorithm employed in the context of thepresent invention as implemented by means of algorithmic unit 11comprising said addition term determining means 13 and additional termincluding means 12 is given by:p=[S ^(h)(Ψ+Ψ_(instability))⁻¹ S] ⁻¹ S ^(h)((Ψ+Ψ_(instability))⁻¹ m.FIG. 3 shows a schematic diagram illustrating a defined region forobviating motion-induced artefacts in accordance with the presentinvention. In FIG. 3, the same reference numerals are used for the sameor similar elements as in FIG. 2. Additionally, in FIG. 3 saiduser-defined region of, for instance, hexagonal shape and area A isdenoted by means of reference numeral 17. The small (cross-like) symbolsinside region 17 of FIG. 3 illustrate individual locations within area A(generally corresponding to said region 17) in terms of image positioncoordinates (x, y). Thus, coil sensitivities s_(i)(x,y) at each of saidlocations (x, y) within region 17 are used to determine the matrixelements Ψ_(instability,ik) of the additional term Ψ_(instability), asdefined above.

In accordance with the present invention, region 17 can be defined priorto acquisition of magnetic resonance data by means of receiving units4.1-4.4. Alternatively, the acquired magnetic resonance raw data isstored in storage means 7, region 17 may also be defined after saidmagnetic resonance data acquisition.

In the embodiment of FIG. 1, input unit 6 is preferably used forproviding geometrical data descriptive of said region 17 to imagereconstruction device 8/algorithmic unit 11 through said secondreceiving means 10.

In all embodiments of the present invention, an image reconstruction mayfirst be performed without including said additional termΨ_(instability) in algorithmic unit 11. Then, following inspection ofthe reconstructed image data, for instance on a displaying unit (notshown) connected with control unit 5, user may identify a moving regionof object 2, e.g. region 17 (heart 16) of the human body 14, such thatimage reconstruction can be reperformed by image reconstruction device8/algorithmic unit 11 involving determining and including the additionalterm Ψ_(instability) through means 12 and 13, respectively, aspreviously described.

For this reason, the approach proposed in the context of the presentinvention can also be referred to as “post-scan REST” which achieves theadvantages of conventional REST (Regional Saturation Technique) withoutsuffering from drawbacks associated with conventional REST, inparticular in terms of scan time, attainable repetition rate andlimitation to saturation geometries in the form of straight slabs. Inthis way, radiation exposure of the object (patient) can be reduced.

In this context, the present invention makes use of the fact thatmagnetic resonance imaging using a plurality of receiving units withdifferent spatial sensitivities allows for intrinsically avoiding imageartefact by proposing a suitable coil combination algorithm (see above)according to which the employed plurality of receiving units, incombination, shows only minimal sensitivity to an area A (region 17 inFIG. 3) which is known to be problematic in the context ofacquiring/reconstructing magnetic resonance images. This approach can beused with both parallel and non-parallel imaging techniques.

FIG. 4 shows a flow chart for illustrating an embodiment of the methodin accordance with the present invention. The method starts with stepS100. In subsequent step S102 a reference scan is performed by means ofa reference coil (not shown) comprised in the system 1 of FIG. 1 inorder to determine the quantity q(x,y), as defined above. Said quantitycan be stored in storage means 7.

Then, in step S104 magnetic resonance raw data is acquired by saidplurality of receiving units by sampling the k-space, as known to aperson skilled in the art. Preferably, the acquired raw data is alsostored in storage means 7.

Then, in step S106 a region of the object, which is expected to move orto have moved, is defined by a user, possibly after inspection of afirst image reconstructed from said raw data, as previously described.Note that in accordance with the invention steps S104 and S106 could bereversed in order, i.e. said region is defined prior toacquiring/storing magnetic resonance data.

Having defined said region, in step S108 the additional term isdetermined in the form of matrix Ψ_(instability), as defined above, andis included in the reconstruction algorithm in subsequent step S110.

Then, in subsequent step S112 a desired magnetic resonance image isreconstructed from the acquired raw data by means of the proposed novelalgorithm, thus effectively obviating the impact of motion artefacts.

The inventive method terminates with step S114.

The invention claimed is:
 1. A magnetic resonance imaging system, comprising: a plurality of receiving units for receiving magnetic resonance signals from an object; an image reconstruction device, said image reconstruction device being adapted to receive magnetic resonance signals of said object from said plurality of receiving units and to perform image reconstruction by combining magnetic resonance signals received by said plurality of receiving units using an image reconstruction algorithm, wherein said image reconstruction device is configured to combine magnetic resonance signal contributions from respective receiving units with spatial receiver coil sensitivities in such a way that a combined sensitivity of the plurality of receiving units to a predetermined spatial region of the object is reduced, wherein the image reconstruction device is further configured to account for motion of said predetermined spatial region for the combining of the magnetic resonance signal contributions from respective receiving units.
 2. The magnetic resonance imaging system of claim 1, wherein said image reconstruction device is further configured to geometrically define said region before or after measuring said magnetic resonance signals.
 3. The magnetic resonance imaging system of claim 1, wherein said image reconstruction device is further configured to estimate image data points (p) by evaluating the expression p=(S ^(h)Ψ′⁻¹ S)⁻¹ S ^(h)Ψ′⁻¹ m, with Ψ′=Ψ+Ψ_(instability), wherein m is a vector with measured magnetic resonance data per receiving unit, S is an array of receiving unit sensitivity matrix, S^(h) is the hermitian transpose of S, Ψ is a noise covariance matrix, and Ψ_(instability) is an additional term accounting for a motion of said predetermined spatial region of the object.
 4. The magnetic resonance imaging system of claim 3, wherein said image reconstruciton deivce is further configured to include said additional term (Ψ_(instability)) in said image reconstruction algorithm.
 5. The magnetic resonance imaging system of claim 3, wherein said image reconstruction device is further configured to determine said additional term (Ψ_(instability)) according to the relation: Ψ_(instability,ik) =a·1/A·Σ _(area) s _(ij)(x,y)s _(kj)(x,y)∥q(x,y)∥², wherein Ψ_(instability,ik) are matrix elements of said additional term, A is a quantity of said predetermined spatial region of the object, q(x,y) is a signal strength of a reference signal, s_(ij)(x,y), s_(k)(x,y) is a sensitivity of receiving unit i, k at image position (x,y), respectively, and α is a predefined numerical factor. 